If you want to do manual SSH key configuration on bitrise.io you can generate an appropriate SSH keypair with a simple Command Line / Terminal command:
ssh-keygen -t rsa -b 4096 -P '' -f ./bitrise-ssh
This will generate two files in the current directory (the directory where you run the command):
You should copy paste the public key to your Git hosting service (GitHub, Bitbucket, etc.), and when you register your app on bitrise.io you’ll have to provide the private key.